Workflows, Workers - Set a concurrency limit for each Workflow

You can now set a concurrency limit on an individual Workflow to cap how many of its instances run at the same time. Use it to protect a database or third-party API that slows down or rejects requests past a certain number of concurrent connections, or to distribute load more evenly across your Workflows so that one high-volume Workflow does not use all of your account's concurrency.

For example, to allow only 10 instances of a Workflow to run at a time, set concurrency.limit on its workflows binding and redeploy:

{
	"workflows": [
		{
			"name": "my-workflow",
			"binding": "MY_WORKFLOW",
			"class_name": "MyWorkflow",
			"concurrency": {
				"limit": 10,
			},
		},
	],
}

[[workflows]]
name = "my-workflow"
binding = "MY_WORKFLOW"
class_name = "MyWorkflow"

  [workflows.concurrency]
  limit = 10

You can also set concurrency on a Workflow declared in exports, or with exports.workflow() in @cloudflare/config.

Instances above the limit are not rejected. They are queued and start automatically as running instances finish or move to the waiting state.

The limit must be a positive integer, and cannot be higher than your account's concurrency limit (100 on Workers Free, 50,000 on Workers Paid). Lowering the limit does not stop instances that are already running. Workflows without a concurrency limit can still use up to your account's full concurrency limit.

Caution

Set concurrency.limit in your configuration on every deploy. If you deploy without it, the Workflow's concurrency limit is removed.

Per-Workflow concurrency limits require Wrangler 4.127.0 or above.

For more information, refer to Concurrency limits.